1. Field of the Invention
The present invention relates semiconductor light emitters, optical transmitting modules, optical transceiver modules, optical communications systems, and a method of manufacturing semiconductor light emitters.
2. Description of the Related Art
In recent years, the amount of information that is exchanged and processed has been increasing exponentially as can be seen in the promulgation of the Internet, and such increase is expected to accelerate further. Because of this, optical fibers are beginning to be used not only in trunk systems but also in communication systems closer to users such as the subscriber systems of individual households and offices and LANs (local area networks), and are also beginning to be used for connections between and inside individual apparatuses, which makes large-capacity information transmission a vital technology.
As a light source for use in such technology, a semiconductor laser of a 1.3-micrometer band or a 1.55-micrometer band is necessary that incurs a little transmission loss on silica fibers and provides good matching characteristics. In order to achieve a market progress for application closer to users, further, communications systems must be provided at low costs.
For the 1.3-micrometer waveband and the 1.55-micrometer waveband, the system of materials used on the InP substrate are typical, and have been reliably used in edge emitting lasers. Such long-wavelength-band semiconductor lasers, however, have drawbacks in that the operating current increases three-folds when an ambient temperature increases from room temperature to 80 degrees Celsius. In order to provide a low-cost system that does not use a cooling device, it is vital to develop long-wavelength-band semiconductor lasers that have better temperature characteristics. The reason why the temperature characteristics are not satisfactory is that electrons are easy to overflow because of the small discontinuity of a conduction band and that this tendency is temperature-sensitive.
The system of materials that can form a 1.3-micrometer-band semiconductor laser on a GaAs substrate has recently been attracting attention. Research has been directed to (Ga)InAs quantum dots, GaAsSb, and GaInNAs (e.g., Patent Document 1). Especially, GaInNAs is recognized as a material that substantially suppresses the temperature dependency of laser characteristics. Here, materials of the GaInNAs system include other Group-III-V elements such as P, Sb, and Al.
GaInNAs is a Group III-V mixed-crystal semiconductor that contains nitrogen (N) and other Group V elements. In GaInNAs, the addition of nitrogen (N) to GaInAs having a larger lattice constant than GaAs provides for the lattice constant to be matched with that of GaAs. Further, band-gap energy is reduced to make it possible to emit light in the 1.3-micrometer band or the 1.55-micrometer band.
In Non-Patent Document 1, for example, Kondo et. al. calculate the band lineup of GaInNAs. GaInNAs has band-gap energy that is reduced by the addition of nitrogen (N). Since the energy of the conduction band and the valence band also lowers, the discontinuity of the conduction band becomes significantly large compared with GaAs lattice-matched materials such as GaInP, AlGaAs, and GaAs. This is expected to provide for a semiconductor laser having superior temperature characteristics. A 10% In composition with a 3% nitrogen composition can produce 1.3-micrometer band. However, there is a problem in that the threshold current density increases rapidly as the nitrogen composition increases.
FIG. 1 is a chart showing a threshold current density that is dependent on the nitrogen composition according to experiments. The horizontal axis represents the percentage (%) of nitrogen composition, and the vertical axis represents the threshold current density. As shown in FIG. 1, the threshold current density increases rapidly with the increase of nitrogen composition. This is because the crystallinity of the GaInNAs layer deteriorates with the increase of nitrogen composition. As a counter measure, the In composition is raised while the nitrogen composition is lowered (e.g., Patent Document 2 and Patent Document 3), with the use of a GaInNAs-system quantum well active layer that has a compressive strain as large as 2% or more relative to the substrate. Based on this construction, some studies (e.g., Non-Patent Document 2) provide a laser device that has a characteristic temperature exceeding 200 K, with the threshold current density of the semiconductor laser being smaller than 1 kA/cm2, and the operating current increasing only 1.3 times even when the ambient temperature rises from room temperature to 80 degrees Celsius.
A GaAs layer is used for the barrier layer. When the GaInNAs-system quantum well active layer having a compressive strain is employed, GaAs is often used for the barrier layer. There are also studies (e.g., Non-Patent Document 3) that report proper temperature characteristics of 140 K-170 K based on the use of GaInAs quantum well active layer having no nitrogen and a high strain. GaAs is used for the barrier layer in this case also.
In the presence of such a high strain, however, there is a need to secure film growth close to a critical film thickness at which three-dimensional growth takes the place of two-dimensional growth. A technique must be contrived for this purpose. Conventionally, a low-temperature growth method (e.g., Patent Document 2) and a method that achieves a surfactant-like effect by adding Sb (e.g., Patent Document 3) are known. These methods, however, limit the design of devices by imposing a limit to the number of quantum wells, for example, for the purpose of suppressing the generation of crystalline defects.
Methods for improvement include using a GadIn1-dNePfAs1-e-f material (e.g., Patent Document 4), a GaNPAs or GaNAs layer (e.g., Patent Document 5), a GaNAs layer (e.g., Patent Document 3), or GaNAsSb (e.g., Non-Patent Document 4) as a barrier layer including N and having a smaller lattice constant than the substrate, thereby providing a strain-compensation structure that suppresses (compensates for) the strain of the active layer. Patent Document 2 teaches a 0.9% N composition in the GaInNAsSb well layers and a 1.8% N composition in the GaNAs barrier layers. Non-Patent Document 4 teaches a 1.7% N composition in the GaInNAsSb well layers and a 2% N composition in the GaNAsSb barrier layers. Patent Document 5 discloses a 2% N composition in both the GaInNAs well layers and the GaNAs barrier layers. In this manner, the N composition in the barrier layers is larger or equal to the N composition in the well layers.
As the reason why a material having added N is used for the barrier layers, Patent Document 4 explains a need to form a strain compensation layer to suppress a strain in the active layer and a need to achieve easy control of an interface between a well layer and a barrier layer. Patent Document 5 explains a need for forming a strain compensation layer to suppress a strain in the active layer. Patent Document 3 explains a need for reducing the discontinuity of a conduction band to lower quantum levels towards longer wavelengths and a need to improve crystallinity by achieving homo-epitaxial through the addition of N to both the well layers and the barrier layers.
The addition of N to GaAs results in the lattice constant being smaller. Moreover, since GaNAs has its quantum level energy lowered by a decrease in the discontinuity of conduction bands relative to the GaInNAs quantum well active layers, an oscillating wavelength becomes longer. Accordingly, N composition necessary for the attainment of a required wavelength can be reduced in the quantum well active layers, making it possible to improve the quality of active layers. However, the same reason (i.e., the decrease in the discontinuity of conduction bands relative to the GaInNAs quantum well active layers) works to create an increasing number of overflowing electrons, resulting in the deterioration of temperature characteristics.
[Patent Document 1] Japanese Patent Application Publication No. 6-37355
[Non-Patent Document 1] Japan. J. Appl. Phys. Vol. 35(1996) pp. 1273-1275
[Patent Document 2] Japanese Patent Application Publication No. 2000-332363
[Patent Document 3] Japanese Patent Application Publication No. 2002-118329
[Non-Patent Document 2] Japan. J. Appl. Phys. Vol. 39 (2000) pp. 3403-3405
[Non-patenting reference 3] IEEE Photon. Technol. Lett. Vol. 12 (2000) pp. 125-127
[Patent Document 4] Japanese Patent Application Publication No. 10-126004
[Patent Document 5] Japanese Patent Application Publication No. 10-145003
[Non-Patent Document 4] Electron. Lett. Vol. 38, pp 277-278 (2002)
Accordingly, there is a need for a semiconductor light emitter, an optical transmission module, an optical transceiver module, and an optical communication system, in which a semiconductor light emitter using a GaInNAs-type quantum well active layer provides satisfactory temperature characteristics and a low threshold.